1. Technical Field
The present disclosure relates to a terminal, a base station, a transmission method, and a reception method.
2. Description of the Related Art
As a mechanism for supporting a future information society, a machine-to-machine (M2M) communication system that achieves services through autonomous communication between devices without asking users to make determinations is expected these years. A smart grid is a specific application of the M2M communication system. The smart grid is an infrastructure system that efficiently supplies a lifeline such as electricity or gas, and performs M2M communication between a smart meter installed in a household or a building and a central server in order to adjust a supply-demand balance of resources autonomously and effectively. Other applications of the M2M communication system include a monitoring system for article management, distance medicine, or the like and remote management of stock or charging of vending machines.
In the M2M communication system, in particular, use of cellular systems having large communication areas is gaining attention. The 3rd Generation Partnership Project (3GPP), which is a standardization group of cellular communication systems, is examining M2M based on a cellular network under a name of machine type communication (MTC) for standardization of long-term evolution (LTE) and LTE-Advanced. In particular, further expansion of communication areas is being examined in consideration of cases where MTC communication devices such as smart meters are provided in places such as basements of buildings and are not available in existing communication areas (e.g., refer to 3GPP TR 36.888 V12.0.0, “Study on provision of low-cost Machine-Type Communications (MTC) User Equipments (UEs) based on LTE,” June 2013). In order to further expand the communication areas, for example, repetition, in which the same signals are transmitted a plurality of times, is being examined.
In a cellular communication system, channels used in uplink, which is communication from a terminal to a base station, are a physical uplink control channel (PUCCH) and a physical uplink shared channel (PUSCH). The PUCCH is a channel for transmitting a response to a downlink data signal transmitted through a physical downlink shared channel (PDSCH), such as a positive response (acknowledgement (ACK)) or a negative response (negative acknowledgement (NACK)) (hereinafter described as an “ACK/NACK”; also referred to as a “response signal”), and control information such as a scheduling request (SR) indicating a request to assign resources. On the other hand, the PUSCH is a channel for transmitting data signals. An ACK/NACK, for example, is 1-bit information indicating either ACK (no error) or NACK (there is an error). PUCCH resources used by the terminal to transmit an ACK/NACK and an SR are secured in advance. In the following description, PUCCH resources used for an ACK/NACK will be referred to as “ACK/NACK resources”, and PUCCH resources used for an SR will be referred to as “SR resources”.
Transmission of ACK/NACK and SR
In Release 11 (hereinafter referred to as “Rel. 11”) of LTE, if transmission of a PUSCH is not assigned to the same subframe as one in which an ACK/NACK is transmitted, the ACK/NACK is transmitted through the PUCCH. In addition, for example, a signal point of binary phase-shift keying (BPSK) is used for an ACK/NACK, and an ACK is transmitted using a signal point of −1, and a NACK is transmitted using a signal point of +1.
FIG. 1 illustrates an example of transmission of an ACK/NACK and an SR using PUCCH resources in Rel. 11. “Information to be transmitted” illustrated in FIG. 1 indicates a signal to be transmitted in each subframe, and “A/N” indicates an ACK/NACK (the same holds in the subsequent drawings).
As illustrated in FIG. 1, if an SR is not transmitted in the same subframe as one in which an ACK/NACK is transmitted, the ACK/NACK is transmitted using an ACK/NACK resource. On the other hand, if transmission of an SR occurs in the same subframe as one in which an ACK/NACK is transmitted, the ACK/NACK is transmitted using an SR resource. In addition, in a subframe in which only transmission of an SR occurs, the SR is transmitted using an SR resource. If only an SR is transmitted, the SR is transmitted using a signal point of +1 (the same signal point as a NACK) in BPSK (e.g., refer to 3GPP TS 36.211 V11.5.0, “Physical channels and modulation (Release 11),” December 2013).
A base station identifies, through blind detection such as a power determination, a resource (an ACK/NACK resource or an SR resource) with which an ACK/NACK is transmitted. If determining that the ACK/NACK has been transmitted using an SR resource, the base station determines that there is an SR and decodes the ACK/NACK using a signal of the SR resource. On the other hand, if determining that the ACK/NACK has been transmitted using an ACK/NACK resource, the base station determines that there is no SR and decodes the ACK/NACK using a signal of the ACK/NACK resource. In addition, if detecting a signal of an SR resource at a timing other than timings (known timings) at which ACK/NACKs are received in response to downlink data signals, the base station determines that there is an SR.
Transmission of ACK/NACK and PUSCH
In Rel. 11, if transmission of a PUSCH is assigned to the same subframe as one in which an ACK/NACK is transmitted, the ACK/NACK is transmitted through the PUSCH.
FIG. 2 illustrates an example of transmission of a PUSCH and an ACK/NACK in Rel. 11. In “information to be transmitted” illustrated in FIG. 2, “Data” indicates an uplink data signal (hereinafter also referred to simply as “data”) (the same holds in the subsequent drawings).
As illustrated in FIG. 2, in a subframe in which only an ACK/NACK is transmitted, the ACK/NACK is transmitted using an ACK/NACK resource. In addition, in a subframe only data is assigned, the data is transmitted using a PUSCH.
In addition, as illustrated in FIG. 2, if data is assigned to the same subframe as one in which an ACK/NACK is transmitted, the ACK/NACK is time-multiplexed with a data signal and transmitted in a PUSCH. More specifically, by puncturing part of a data signal mapped in a resource adjacent to a reference signal (RS), the ACK/NACK is arranged in the resource for that part (e.g., refer to 3GPP TS 36.212 V11.4.0, “Multiplexing and channel coding (Release 11),” December 2013).
The base station determines whether an ACK/NACK is included in a received PUSCH through blind detection. Here, the base station can detect a timing at which an ACK/NACK is transmitted in response to a downlink data signal (PDSCH) on the basis of assignment of the downlink data signal in a physical downlink control channel (PDCCH). The base station can therefore decode the PUSCH while assuming that an ACK/NACK is included, without performing blind detection in a subframe in which the terminal must transmit the ACK/NACK. Due to the following reason, however, the base station determines presence or absence of an ACK/NACK through blind detection. If the terminal fails to receive a PDCCH with which the terminal is notified of assignment of a downlink data signal, the terminal does not transmit an ACK/NACK but transmits only a data signal using a PUSCH. At this time, the PUSCH includes only the data signal, but if the base station decodes the PUSCH while assuming that the PUSCH includes an ACK/NACK, data signal decoding properties deteriorate. The base station therefore, initially needs to determine whether an ACK/NACK is included.
Repetition
In Rel. 11, ACK/NACK repetition in the PUCCH in which the maximum number of repetitions is six is introduced. FIG. 3 illustrates an example of ACK/NACK repetition and SR transmission in PUCCH resources in Rel. 11.
An ACK/NACK repetition is transmitted using ACK/NACK resources secured in advance. In addition, as illustrated in FIG. 3, if transmission of an SR occurs in the same subframe as one in which transmission of an ACK/NACK repetition, priority is given to the transmission of the ACK/NACK repetition using an ACK/NACK resource, and the SR is dropped (not transmitted) (e.g., refer to 3GPP TS 36.213 V11.5.0, “Physical layer procedures (Release 11),” December 2013).